Modulation of parathyroid hormone receptor signal transduction pathways in osteoblasts.
Published: 2005
Language: eng
Number of pages: 212
Description: Parathyroid hormone (PTH) is one of the major regulators of bone and mineral homeostasis. The effects of PTH on osteoblasts are mediated by binding to PTH1 receptors, which couple to both Gsalpha and Gq/11 alpha to stimulate adenylyl cyclase (AC) and phospholipase C (PLC) signaling pathways, respectively. Glucocorticoid treatment significantly increased the PTH-stimulated PLC response in osteoblasts by up-regulating the G q/11alpha expression. The first goal of my thesis was to determine the mechanism of regulation of Gq/11alpha expression by glucocorticoids in rat osteoblastic UMR 106-01 cells. The results demonstrated that G 11alpha is the major Gq/11alpha subtype expressed in UMR cells and that glucocorticoids significantly increase G11alpha protein expression. This increase is mediated primarily by decreasing the rate of protein degradation but also by increasing gene transcription. Since glucocorticoids have been reported to regulate the expression of a number of genes in osteoblasts, it was important to determine whether increased G 11alpha to could change the PTH responses in osteoblasts independently of glucocorticoids. A transient transfection method with G11alpha cDNA was used specifically to increase G11alpha protein expression in UMR cells. Increasing G11alpha expression alone augmented both basal and PTH-stimulated PLC activity. In addition, PTH-mediated signaling responses downstream of the G11alpha-coupled PLC pathway, including basal expression and PTH-stimulated membrane translocation of protein kinase C (PKC) isoforms as well as expression of matrix metalloproteinase-13 (MMP-13) gene, were up-regulated by increasing G11alpha expression. These effects of increased G11alpha expression in osteoblasts may contribute to glucocorticoid-induced osteoporosis. In addition to glucocorticoids, 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) is another hormone that modulates PTH responses in osteoblasts. 1,25(OH)2D3 attenuates PTH responses by inhibiting AC activity, but the mechanism of action was not clear. I set out to elucidate the mechanism by which 1,25(OH) 2D3 inhibits PTH-stimulated AC activity in UMR cells. The results indicated that AC VI is the dominant AC subtype expressed in UMR cells and 1,25(OH)2D3 can activate specific PKC isoforms that lead to phosphorylation of AC VI and inhibition of PTH-stimulated AC activity in osteoblasts. Overall, I demonstrate that PTH signal transduction can be altered by several mechanisms in response to two hormones known to affect osteoblast function.
Notes: Thesis (Ph.D.)--University of Toronto, 2005.

Electronic version licensed for access by U. of T. users.

Source: Dissertation Abstracts International, Volume: 66-06, Section: B, page: 3079.